Bone formation by osteoblasts and bone resorption by osteoclasts must be tightly regulated to maintain skeletal homeostasis. Pathogenesis arises when the balance between these opposing processes is disturbed. In osteoporosis, for example, bone resorption outweighs formation. Detailed understanding of the mechanisms by which hormones and other molecules stimulate resorption is lacking. It is known, however, that many of these resorptive agents stimulate osteoclast activity only when osteoblasts are also present. Parathyroid hormone, 1,25 dihydroxyvitamin D3, interleukin 1, tumor necrosis factor, and transforming growth factor beta all appear to act in this indirect fashion. These findings have led to the hypothesis that osteoclast activity is controlled in large part by changes in production of cytokines by osteoblasts. This hypothesis suggests that the antiresorptive effects of estrogen may be due to it's ability to block cytokine production induced by the resorptive agents. The goals of this proposal are therefore to determine (1) by reverse transcription-polymerase chain reaction, bioassays, and ELISAs which osteoblast-derived cytokines are affected by the resorptive agents and by estrogen, (2) the identity and functions of the cytokines that are involved in regulating resorption in osteoblast-osteoclast co-cultures, and (3) the intracellular mechanisms responsible for changes in osteoblast cytokine production. These studies will define cytokine networks fundamental to normal physiological states and will thus provide information that is crucial for understanding pathophysiological states that are characterized by changes in bone mass.